Tone bursts are used in some communication networks to transmit simple, low data-rate signalling messages. A major reason for using a tone is that the required detection and generation circuitry may be relatively simple. A tone burst may be used, for example, to initiate and synchronize activation across a high data-rate network. In particular, a tone is used by a telecommunications U-interface transceiver at one end of an Integrated Services Digital Network (ISDN) U-interface node to signal to a distant end that the transceiver is requesting activation. By using a tone instead of the normal data signal using the 2B1Q line code of the U-interface transceiver, a U-interface transceiver may detect activation requests without requiring adaptive equalization or timing synchronization. Another advantage of using tones is that tones can be distinguished from cross talk and impulse noise in the U-interface, thus preventing false attenuation attempts.
An important technical characteristic of tone detectors is that the tone detector must be able to detect a transmitted tone under worst-case transmission conditions with a minimum probability of error. Known tone detectors are typically implemented in analog circuitry with components which may have varying characteristic over temperature and process changes. Tone detectors which have been implemented in digital circuitry include phase locked loop circuits, zero crossing and interval measurement circuits and circuits which threshold detect the normalized energy output of a bandpass filter. In order to guarantee reliable tone detection, a large amount of digital circuitry is typically required for these implementations.